U.S. patent application number 09/925223 was filed with the patent office on 2003-05-15 for forming ferroelectric pb (zr, ti)o3 films.
Invention is credited to Aggarwal, Sanjeev, Gilbert, Stephen R., Hunter, Stevan, Singh, Kaushal.
Application Number | 20030091740 09/925223 |
Document ID | / |
Family ID | 25451416 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030091740 |
Kind Code |
A1 |
Gilbert, Stephen R. ; et
al. |
May 15, 2003 |
Forming ferroelectric Pb (Zr, Ti)O3 films
Abstract
Improved methods of forming PZT thin films that are compatible
with industry-standard chemical vapor deposition production
techniques are described. These methods enable PZT thin films
having thicknesses of 70 nm or less to be fabricated with high
within-wafer uniformity, high throughput and at a relatively low
deposition temperature. In one aspect, a source reagent solution
comprising a mixture of a lead precursor, a titanium precursor and
a zirconium precursor in a solvent medium is provided. The source
reagent solution is vaporized to form a precursor vapor. The
precursor vapor is introduced into a chemical vapor deposition
chamber containing the substrate. In another aspect, before
deposition, the substrate is preheated during a preheating period.
After the preheating period, the substrate is disposed on a heated
susceptor during a heating period, after which a PZT film is formed
on the heated substrate.
Inventors: |
Gilbert, Stephen R.; (San
Francisco, CA) ; Singh, Kaushal; (Santa Clara,
CA) ; Aggarwal, Sanjeev; (Plano, TX) ; Hunter,
Stevan; (Fort Collins, CO) |
Correspondence
Address: |
AGILENT TECHNOLOGIES
Legal Department, 51U-PD
Intellectual Property Administration
P.O. Box 58043
Santa Clara
CA
95052-8043
US
|
Family ID: |
25451416 |
Appl. No.: |
09/925223 |
Filed: |
August 8, 2001 |
Current U.S.
Class: |
427/255.28 ;
257/E21.272; 427/126.3 |
Current CPC
Class: |
H01L 21/02197 20130101;
H01L 21/02205 20130101; H01L 21/31691 20130101; C23C 16/409
20130101; Y10S 427/101 20130101; H01L 21/02271 20130101 |
Class at
Publication: |
427/255.28 ;
427/126.3 |
International
Class: |
C23C 016/00; B05D
005/12 |
Claims
What is claimed is:
1. A method of forming a ferroelectric PZT film on a substrate,
comprising: providing a premixed source reagent solution comprising
a mixture of a lead precursor, a titanium precursor and a zirconium
precursor in a solvent medium; vaporizing the source reagent
solution to form a precursor vapor; and introducing the precursor
vapor into a chemical vapor deposition chamber containing the
substrate.
2. The method of claim 1, wherein the zirconium precursor comprises
Zr(OiPr).sub.2(thd).sub.2 or Zr(thd).sub.4 or
Zr(O.sup.tBu).sub.2(thd).su- b.2.
3. The method of claim 1, wherein the lead precursor is
Pb(thd).sub.2(pmdeta), the zirconium precursor is
Zr(OiPr).sub.2(thd).sub- .2, and the titanium precursor is
Ti(OiPr).sub.2(thd).sub.2.
4. The method of claim 1, wherein the lead precursor, the titanium
precursor and the zirconium precursor have a combined concentration
between about 0.05 M and about 1.0 M in solution.
5. The method of claim 1, wherein the source reagent solution is
characterized by lead, zirconium and titanium concentrations
between about 5% and 95%.
6. The method of claim 1, further comprising introducing into the
chemical vapor deposition chamber an oxidizing co-reactant gas
comprising 5-100% N.sub.2O.
7. The method of claim 6, wherein the oxidizing co-reactant gas
comprises 50-75% N.sub.2O.
8. The method of claim 1, further comprising introducing into the
chemical vapor deposition chamber an oxidizing co-reactant gas
comprising one or more of the following gases: N.sub.2O, O.sub.2,
and O.sub.3.
9. The method of claim 1, further comprising: providing a second
premixed source reagent solution comprising a second mixture of the
lead precursor, the titanium precursor and the zirconium precursor
in the solvent medium, wherein the first source reagent mixture is
different from the second source reagent mixture; mixing the first
and second reagent solutions to form a precursor solution; and
vaporizing the precursor solution to form the precursor vapor.
10. The method of claim 9, wherein the first and second source
reagent solutions are characterized by a lead concentration in a
range of about 28-65%, a zirconium concentration in a range of
about 14-29%, and a titanium concentration in a range of about
20-43%.
11. The method of claim 1, wherein the solvent medium comprises an
octane-based solvent.
12. The method of claim 1, wherein the source reagent solution is
vaporized at a temperature in the range of about 180-210.degree.
C.
13. The method of claim 1, further comprising maintaining the
chemical vapor deposition chamber at a pressure in the range of
about 0.5-10 torr during deposition.
14. The method of claim 13, wherein the chemical vapor deposition
chamber is maintained at a pressure in the range of about 0.5-4
torr during deposition.
15. The method of claim 14, wherein the chemical vapor deposition
chamber is maintained at a pressure of approximately 4 torr during
deposition.
16. The method of claim 1, wherein the source reagent solution is
selected to obtain a precursor vapor having a Zr/(Zr+Ti) ratio in
the range of about 0.05-0.70.
17. The method of claim 1, wherein the source reagent solution is
selected to obtain a precursor vapor having a Pb/(Zr+Ti) ratio in
the range of about 0.3-3.0.
18. The method of claim 1, further comprising preheating the
substrate during a preheating period.
19. The method of claim 18, wherein the preheating period is about
5-30 seconds long.
20. The method of claim 18, further comprising disposing the
preheated substrate on a heated susceptor during a heating period
prior to formation of the PZT film on the substrate.
21. The method of claim 20, wherein the heating period is about
30-60 seconds long or longer.
22. The method of claim 1, further comprising providing a flow of a
purge gas to reduce film depositions on susceptor and chamber wall
surfaces.
23. A method of forming a ferroelectric PZT film on a substrate,
Comprising: introducing a substrate into a chemical vapor
deposition chamber; preheating the substrate during a preheating
period; after the preheating period, disposing the substrate on a
heated susceptor during a heating period; forming a precursor
solution from a mixture of a lead precursor, a titanium precursor
and a zirconium precursor in a solvent medium; vaporizing the
precursor solution to form a precursor vapor; and introducing the
precursor vapor into the chemical vapor deposition chamber to form
a ferroelectric PZT film on the heated substrate.
24. The method of claim 23, wherein the substrate is preheated by
supporting the substrate above the heated susceptor during the
preheating period.
25. The method of claim 23, further comprising providing a flow of
a purge gas to reduce film depositions on susceptor and chamber
wall surfaces.
Description
TECHNICAL FIELD
[0001] This invention relates to systems and methods of forming
ferroelectric Pb(Zr,Ti)O.sub.3 (PZT) films, including ferroelectric
PZT films for use in ferroelectric random access memory
devices.
BACKGROUND
[0002] Today, several trends exist in the semiconductor device
fabrication industry and the electronics industry that are driving
the development of new material technologies. First, devices are
continuously getting smaller and smaller and requiring less and
less power. A reason for this is that more personal devices are
being fabricated which are very small and portable, thereby relying
on a small battery as its supply source. For example,
cellular-phones, personal computing devices, and personal sound
systems are devices that are in great demand in the consumer
market. Second, in addition to being smaller and more portable,
personal devices are requiring more computational power and on-chip
memory. In light of these trends, there is a need in the industry
to provide a computational device that has a fair amount of memory
and logic functions integrated onto the same semiconductor chip.
Preferably, this computation device will include a non-volatile
memory so that if the battery dies, the contents of the memory will
be retained. Examples of conventional non-volatile memories include
electrically erasable, programmable read only memories and flash
EEPROMs.
[0003] A ferroelectric memory (FeRAM) is a non-volatile memory that
utilizes a ferroelectric material as a capacitor dielectric
situated between a bottom electrode and a top electrode.
Ferroelectric materials, such as SrBi.sub.2Ta.sub.2O.sub.9 (SBT)
and Pb(Zr,Ti)O.sub.3 (PZT), are being used in the fabrication of a
wide variety of memory elements, including ferroelectric random
access memory (FeRAM) devices. In general, ferroelectric memory
elements are non-volatile because of the bistable polarization
state of the material. In addition, ferroelectric memory elements
may be programmed with relatively low voltages (e.g., less than 5
volts) and are characterized by relatively fast access times (e.g.,
less than 40 nanoseconds) and operational robustness over a large
number of read and write cycles. These memory elements also consume
relatively low power, may be densely packed, and exhibit radiation
hardness.
[0004] Recent efforts to develop fabrication processes for
ferroelectric materials have focused on integrating FeRAM
technology with semiconductor integrated circuit technology.
Accordingly, such efforts have focused on scaling FeRAM capacitor
areas, cell sizes and operating voltages downward in accordance
with the scale of current integrated circuit dimensions. In
addition to small lateral dimensions (i.e., dimensions parallel to
the film surface), the ferroelectric dielectric must be relatively
thin and must have a relatively low coercive field to achieve FeRAM
devices having low operating voltages.
[0005] Recently, PZT has been demonstrated to be scalable to
relatively small lateral dimensions and low operating voltages. For
example, International Patent Publication No. WO 00/49646 discloses
a process for forming a scalable PZT material by liquid delivery
metalorganic chemical vapor deposition (MOCVD) without PZT film
modification techniques, such as acceptor doping or use of film
modifiers (e.g., Nb, Ta, La, Sr, Ca, and the like). In accordance
with this process, liquid precursor solutions of the component
metals are mixed and flash vaporized. The resulting source reagent
vapor is introduced into a chemical vapor deposition chamber where
the PZT film is deposited on a substrate. In one embodiment, the
metalorganic precursors are lead bis(2,2,6,6-tetramethyl--
3,5-heptanedionate) (hereinafter "Pb(thd).sub.2") as a Pb
precursor, titanium
bis(isopropoxide)bis(2,2,6,6-tetramethyl-3,5-heptanedionate)
(hereinafter "Ti(O-i-Pr).sub.2,(thd).sub.2") as a Ti precursor, and
zirconium tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionate)
(hereinafter "Zr(thd).sub.4") as a Zr precursor In another
embodiment, the lead precursor is lead
bis(2,2,6,6-tetramethyl-3,5-heptanedionate) N, N', N"-pentamethyl
diethylenetriamine (hereinafter "Pb(thd).sub.2 pmdeta") and the
zirconium precursor is zirconium bis(isopropoxide)bis(2,2,6.6-tet-
ramethyl-3,5-heptanedionate) (hereinafter
"Zr(O-i-Pr).sub.2(thd).sub.2"). The solvent media used in the
liquid delivery MOCVD process is selected to be compatible with the
specific metalorganic precursors used for forming the PZT thin film
materials and efficacious in the constituent liquid delivery and
CVD process steps. Illustrative multi-component solvent
compositions include: tetrahydrofuran: isopropanol: tetraglyme in a
8:2:1 volume ratio; octane: decane: polyamine in a 5:4:1 volume
ratio; and octane: polyamine in a 9:1 volume ratio. According to
the WO 00/49646 patent publication, the resulting PZT material is
pulse length scalable or E-field scalable, or both, and is useful
for ferroelectric capacitors having dielectric thicknesses that
range from about 20 nanometers to about 150 nanometers and having
lateral dimensions that extend down to as small as 0.15
micrometers.
[0006] Chemical vapor deposition (CVD) is a particularly attractive
method for forming thin PZT films because CVD is readily scaled up
to production runs and because CVD technology is sufficiently
mature and developed that CVD may be applied readily to new film
processes. In general, CVD requires that the element source
reagents (i.e., the precursor compounds and complexes containing
the elements or components of interest) must be sufficiently
volatile to permit gas phase transport into the chemical vapor
deposition reactor. The elemental component source reagents should
decompose in the CVD reactor for deposition on the desired
substrate surface at the desired growth temperatures. Premature gas
phase reactions leading to particulate formation should be avoided.
In addition, the source reagents should not decompose in the
transport lines before reaching the reactor deposition chamber. In
sum, in order to deposit CVD films having desirable properties, the
stoichiometry and other process conditions must be controlled for a
given baseline chemistry to create a transport window that enables
component materials to combine on a substrate in a desired way.
SUMMARY
[0007] The invention features improved methods of forming PZT thin
films that are compatible with industry-standard chemical vapor
deposition production techniques. The invention enables PZT thin
films having thicknesses of 70 nm or less to be fabricated with
high within-wafer uniformity, high throughput and at a relatively
low deposition temperature.
[0008] In one aspect, the invention features a method of forming a
ferroelectric PZT film on a substrate. In accordance with this
method, a premixed source reagent solution comprising a mixture of
a lead precursor, a titanium precursor and a zirconium precursor in
a solvent medium is provided. The source reagent solution is
vaporized to form a precursor vapor. The precursor vapor is
introduced into a chemical vapor deposition chamber containing the
substrate.
[0009] Embodiments of the invention may include one or more of the
following features.
[0010] The zirconium precursor preferably comprises
Zr(OiPr).sub.2(thd).sub.2 or Zr(thd).sub.4 or
Zr(O.sup.tBu).sub.2(thd).su- b.2. In one embodiment, the lead
precursor is Pb(thd).sub.2(pmdeta), the zirconium precursor is
Zr(OiPr).sub.2(thd).sub.2, and the titanium precursor is
Ti(OiPr).sub.2(thd).sub.2. The lead precursor, the titanium
precursor and the zirconium precursor preferably have a combined
concentration between about 0.05 M and about 1.0 M in solution. The
source reagent solution preferably is characterized by lead,
zirconium and titanium concentrations between about 5% and 95%.
[0011] In some embodiments, an oxidizing co-reactant gas comprising
5-100% N.sub.2O and, more preferably 50-75% N.sub.2O, is introduced
into the chemical vapor deposition chamber. The oxidizing
co-reactant gas also may include O.sub.2 or O.sub.3, or both.
[0012] In some embodiments, a second source reagent solution
comprising a second premixed mixture of the lead precursor, the
titanium precursor and the zirconium precursor in the solvent
medium is provided. The first source reagent mixture preferably is
different from the second source reagent mixture. The first and
second reagent solutions are mixed to form a precursor solution,
and the precursor solution is vaporized to form the precursor
vapor. In one embodiment, the first and second source reagent
solutions preferably are characterized by a lead concentration in a
range of about 28-65%, a zirconium concentration in a range of
about 14-29%, and a titanium concentration in a range of about
20-43%.
[0013] The solvent medium preferably comprises an octane-based
solvent.
[0014] The source reagent solution may be vaporized at a
temperature in the range of about 180-210.degree. C. During
deposition, the chemical vapor deposition chamber preferably is
maintained at a pressure in a range of about 0.5-10 torr and, more
preferably, in a range of about 0.5-4 torr and, still more
preferably, at a pressure of approximately 4 torr. The source
reagent solution preferably is selected to obtain a precursor vapor
having a Zr/(Zr+Ti) ratio in the range of about 0.05-0.70 and a
Pb/(Zr+Ti) ratio in the range of about 0.3-3.0.
[0015] The substrate preferably is preheated during a preheating
period prior to disposing the substrate on the susceptor. The
preheating period may be about 5-30 seconds long. The preheated
substrate may be deposited on a heated susceptor during a heating
period prior to formation of the PZT film on the substrate. The
heating period may be about 30-60 seconds long or longer. A flow of
a purge gas may be provided to reduce film depositions on the
susceptor and chamber wall surfaces. In some embodiments, in
addition to the purge gas flow, the pre-heating and heating process
steps may be performed in a gas flow that includes a combination of
one or more of the following gases: O.sub.2, N.sub.2O, O.sub.3, and
an inert gas (e.g., He, N.sub.2, or Ar).
[0016] Other features and advantages of the invention will become
apparent from the following description, including the drawings and
the claims.
DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a diagrammatic representation of a chemical vapor
deposition system for forming PZT films.
[0018] FIG. 2 is a flow diagram of a method of forming PZT
films.
[0019] FIG. 3 is a graphical representation of the baseline
chemistry for two source reagent precursor mixtures.
[0020] FIG. 4A is a diagrammatic representation of a substrate
being supported above a heated susceptor during a preheating period
prior to the deposition of a PZT film on the substrate.
[0021] FIG. 4B is a diagrammatic representation of the substrate of
FIG. 4A disposed on the heated susceptor during a heating period
prior to the deposition of a PZT film on the substrate.
[0022] FIG. 5 is a graph of the Pb/(Ti+Zr) ratio and the Zr/(Zr+Ti)
ratio in PZT films formed by the method of FIG. 2 plotted as a
function of the precursor vapor Pb/(Ti+Zr) ratio.
[0023] FIG. 6 is a graph of z-range plotted as a function of PZT
film thickness as measured by atomic force microscopy (AFM) for PZT
films deposited in accordance with the baseline chemistry and
process conditions described herein.
[0024] FIG. 7 is a graph of the Pb/(Ti+Zr) ratio in PZT films
formed by the method of FIG. 2 plotted as a function of the
precursor vapor Pb/(Ti+Zr) ratio for four different CVD chamber
pressures.
[0025] FIG. 8 is a graph of the Pb/(Ti+Zr) ratio in PZT films
formed by the method of FIG. 2 plotted as a function of the
precursor vapor Pb/(Ti+Zr) ratio for an oxidizer gas comprising
100% O.sub.2 and an oxidizer gas comprising 50% O.sub.2 and 50%
N.sub.2O.
[0026] FIG. 9 is a graph of the Pb/(Ti+Zr) ratio in PZT films
formed by the method of FIG. 2 plotted as a function of the
precursor vapor Pb/(Ti+Zr) ratio at a CVD chamber pressure of 2
torr and an oxidizer gas comprising 50% O.sub.2 and 50% N.sub.2O
for two different wafer temperatures and constant susceptor
temperature of 630.degree. C.
[0027] FIG. 10 is a graph of the Pb/(Ti+Zr) ratio in PZT films
formed by the method of FIG. 2 plotted as a function of the
precursor vapor Pb/(Ti+Zr) ratio at a CVD chamber pressure of 2
torr and an oxidizer gas comprising 25% O.sub.2 and 75% N.sub.2O
for two different wafer temperatures and constant susceptor
temperature of 620.degree. C.
DETAILED DESCRIPTION
[0028] In the following description, like reference numbers are
used to identify like elements. Furthermore, the drawings are
intended to illustrate major features of exemplary embodiments in a
diagrammatic manner. The drawings are not intended to depict every
feature of actual embodiments nor relative dimensions of the
depicted elements, and are not drawn to scale.
[0029] Referring to FIG. 1, in one embodiment, a system 10 for
forming PZT films by liquid delivery metalorganic chemical vapor
deposition includes a chemical vapor deposition (CVD) chamber 12
that is coupled to a dual precursor ampoule liquid delivery system
14 and a single vaporizer 16. CVD chamber 12 may be a 200 mm MOCVD
Giga-Cap.TM. chamber, which is available from Applied Materials,
Inc. of Santa Clara, Calif., U.S.A. CVD chamber 12 includes a gas
distribution manifold 18 and a showerhead 20 that is configured to
introduce PZT precursor vapor into CVD chamber 12 from which a PZT
film may be formed on an exposed surface of a substrate 22, which
is supported on a heated susceptor 24. In one embodiment, the
spacing between showerhead 20 and susceptor 24 is approximately
7.5-10 mm, and preferably is approximately 8.9 mm. The exposed
surface of substrate 22 may correspond to the top surface of a
silicon wafer, a layer of silicon dioxide formed on a silicon
wafer, gallium arsenide, magnesium oxide, sapphire, or the top
surface of a multilayer structure that includes, for example, a
complex integrated circuit that is formed on a semiconductor wafer.
In one embodiment, substrate 22 includes a multilayer bottom
electrode structure of Ir (100 nm)/TiAlN (100
nm)/Si.sub.3N.sub.4/SiO.sub.2 that is formed on a silicon wafer. In
another embodiment, substrate 22 includes a multilayer bottom
electrode structure of IrO.sub.x (50 nm)/Ir (50 nm)/TiAlN (100
nm)/Si.sub.3N.sub.4/SiO.sub.2 that is formed on a silicon
wafer.
[0030] Liquid delivery system 14 includes a solvent ampoule 26 and
a pair of source reagent ampoules 28, 30 containing respective
metalorganic mixtures of the component metals needed to form PZT
films. Solvent and source reagent ampoules 26-30 are coupled to
respective liquid flow controllers 32, 34, 36, which are configured
to meter precise quantities of fluid into an equal number of
manifolds 38, 40, 42. The metered solvent and metalorganic mixtures
are delivered to a final mixing chamber 44 where they are mixed to
form a liquid PZT precursor composition. The liquid PZT precursor
composition is introduced into vaporizer 16 where the liquid is
vaporized, for example, by flash vaporization on a vaporization
element (e.g., a porous frit element or a wire grid) that is heated
to a suitable temperature to form a precursor vapor. A gas flow
controller 46 controls the flow of a carrier gas (e.g., argon gas
or helium gas), which transports the precursor vapor into CVD
chamber 12 through a valve 47. An additional push gas source (e.g.,
argon or helium) also may be connected directly to vaporizer 16
through a gas flow controller 45. Gas flow controllers 48, 49, 50
meter precise quantities of oxidizing co-reactant gases (e.g.,
O.sub.2, O.sub.3, N.sub.2O, or a combination of one or more of
these gases) into gas distribution manifold 18, where the oxidizing
gases mix with the precursor vapor before being introduced into CVD
chamber 12.
[0031] Referring to FIGS. 1, 2, 3, 4A, 4B and 5, and initially to
FIGS. 1 and 2, a PZT film may be formed on substrate 22 as
follows.
[0032] During a PZT deposition, a gas flow controller 60 introduces
a flow of a purge gas (e.g., argon gas or helium) into CVD chamber
12 to reduce film depositions on the inner wall surfaces of CVD
chamber 12 and susceptor 24 (step 63). In one embodiment, the purge
gas flow rate is about 250 sccm. During the PZT deposition, the
purge gas flow assists in the removal of unconsumed gas molecules,
partially reacted compounds and reactive byproducts from CVD
chamber 12 through a valve 51, which is coupled to an evacuation
system (or "vacuum foreline") 52. Evacuation system 52 includes
several cold traps 54, 56 and 58.
[0033] The solvent and metalorganic mixtures contained in ampoules
26-30 are mixed to form a PZT precursor solution (step 64). As
mentioned above, source reagent ampoules 28, 30 contain different
premixed, concentrated solutions of a lead precursor, a titanium
precursor and a zirconium precursor in a solvent medium. In one
embodiment, the lead precursor is Pb(thd).sub.2(pmdeta), the
zirconium precursor is Zr(OiPr).sub.2(thd).sub- .2, and the
titanium precursor is Ti(OiPr).sub.2(thd).sub.2. The solvent
preferably is an octane-based solvent (e.g., a "G" solvent
containing an octane:decane:polyamine mixture in a volume ratio of
5:4:1 and available from Applied Technology Materials, Inc. of
Danbury, Conn. U.S.A). Other embodiments may include mixtures of
different Pb, Zr and Ti precursors and solvent systems. In general,
the precursors should exhibit good ambient stability, high
volatility and good thermal compatibility. For example, in some
embodiments, the Zr(OiPr).sub.2(thd).sub.2 zirconium precursor may
be replaced by Zr(OiPr).sub.6(thd).sub.2 or Zr(thd).sub.4 or
Zr(O.sup.tBu).sub.2(thd).sub.2. Each of these precursors is
available from Applied Technology Materials, Inc. of Danbury, Conn.
U.S.A.
[0034] It has been found that the use of premixed solutions that
contain each of the metal precursors enhances the run-to-run
repeatability and the throughput of the PZT film forming process
relative to processes in which elemental precursors or incomplete
precursor mixtures are used. The use of such complete precursor
mixtures also enables the use of a single vaporizer, which
simplifies the system design. In addition, the use of two such
complete precursor mixtures allows the size of the composition
space from which precursor solutions may be formed to be reduced
substantially, further improving the repeatability of the process
while providing sufficient flexibility for process designers to
optimize the baseline chemistry to achieve a desired film
composition for a given set of process parameters.
[0035] As shown in FIG. 3, in some embodiments, the metalorganic
precursor composition space includes lead, zirconium and titanium
concentrations in the range of 5-95%. In one embodiment, the lead
concentration is in the range of about 28%-65%, the zirconium
concentration is in the range of about 14%-29%, and the titanium
concentration is in the range of about 20%-43%. The lead precursor,
titanium precursor and zirconium precursor have combined
concentration of about 0.05-0.5 M in solution and, more preferably,
have a combined concentration of about 0.2-0.35 M in solution. In
one preferred embodiment, ampoules 28 and 30 each contains
Pb(thd).sub.2(pmdeta), Zr(OiPr).sub.2(thd).sub.2, and
Ti(OiPr).sub.2(thd).sub.2 with the following respective component
metal concentrations:
1 TABLE 1 Pb Zr Ti Total Concen- Concen- Concen- Concen- tration
tration tration tration Ampoule (Molar) (Molar) (Molar) (Molar) Low
28 0.090 0.090 0.135 0.315 Pb (28.6%) (28.6%) (42.8%) High 30 0.205
0.045 0.066 0.316 Pb (64.9%) (14.2%) (20.9%)
[0036] In this embodiment, the reagent solution flow from ampoule
28 is approximately 65-82 mg/minute and the reagent solution flow
from ampoule 30 is approximately 118-135 mg/minute for a total
reagent solution flow of approximately 200 mg/minute.
[0037] After the reagent solutions have been mixed to form the
precursor solution, the precursor solution is vaporized to form a
precursor vapor (step 66). For the above-described baseline
chemistry, the vaporizer temperature preferably is in the range of
180-210.degree. C. and, more preferably, is about 190.degree. C.
The jackets, lids and other feedthrough apparatus preferably are
maintained at the same temperature as vaporizer 16. The carrier gas
transports the precursor vapor from vaporizer 16. In one
embodiment, the carrier flow through vaporizer 16 is about 250
sccm. Initially, the precursor vapor is diverted to the evacuation
system 52 (step 68).
[0038] Referring to FIGS. 4A and 4B, while the precursor vapor is
being diverted to the evacuation system 52, substrate 22 is loaded
onto lift pins 62 inside CVD chamber 12 (step 70). Lift pins 62 are
configured to support substrate 22 above heated susceptor 24 during
a preheating period in which substrate 22 is heated indirectly by
susceptor 24 (e.g., by radiative and convective heating) (step 72).
The preheating process allows substrate 22 to be heated gradually
and, thereby, substantially reduces the incidence of thermal shock
that otherwise might occur if substrate 22 were placed immediately
into contact with susceptor 24. Such thermal shock might cause
substrate 22 to break inside CVD chamber 12, in which case CVD
chamber 12 would have to be opened and cleaned, a process that
substantially reduces the productivity of the system. In one
embodiment, the preheating period is about 5-30 seconds long.
[0039] After the preheating period has expired (step 74), substrate
22 is lowered into contact with heated susceptor 24 (step 76). In
general, substrate 22 may be heated to a final processing
temperature of 450-610.degree. C., which is a suitable temperature
range for forming a PZT film from the metal constituents of the
precursor vapor. In one embodiment, during a heating period,
susceptor 24 is set to a temperature of about 640-650.degree. C.
and heats substrate 22 to a final processing temperature of
approximately 600-609.degree. C. In one embodiment, the heating
period is about 30-60 seconds long or longer.
[0040] After the heating period has expired (step 78), the PZT
precursor vapor is mixed with oxidizing co-reactant gases (e.g.,
O.sub.2, O.sub.3, N.sub.2O, or a combination of one or more of
these gases) and the gas/vapor mixture is introduced into CVD
chamber 12 to form a PZT film on the exposed surface of substrate
22 (step 80) until a desired PZT film thickness has been deposited
(step 81). During the PZT deposition, CVD chamber 12 preferably is
maintained at a pressure of about 0.5-10 torr and, more preferably,
is maintained at a pressure of about 4 torr. It has been observed
that the PZT film deposition rate increases with chamber pressure
over the pressure range of 0.5-10 torr. Indeed, the deposition
rates in this pressure range are substantially greater than the
deposition rates that are achieved at lower chamber pressures
(e.g., below 1 torr). Under the above-described preferred
deposition conditions, the deposition rate is approximately 12-20
nm/minute and, in one embodiment, the deposition rate is about 16
nm/minute. In general, the film composition should be tuned so that
it falls within the self-correcting Pb composition regime. In
addition, within the self-correcting region, the film properties
vary significantly with precursor concentration despite the fact
that the film remains stoichiometric. Consequently, within the
self-correcting regime, the Pb/(Zr+Ti) (gas) composition should be
chosen to optimize film properties. The range of Pb/(Zr+Ti) ratios
corresponding to the self-correcting region are dependent on
various process conditions, including pressure, substrate
temperature, and oxidizer gases.
[0041] As shown in FIG. 5, for the above-described preferred
baseline chemistry and process conditions (which are summarized
below in Table 2), a process window yielding single-phase PZT
exists that corresponds to a self-correcting Pb composition regime
that is characterized by precursor vapor Pb/(Zr+Ti) (gas) ratios
between 0.8 and 1.3. Consequently, the starting precursor solutions
are chosen so that this range of compositions is easily accessible.
As shown, within this process window, the Zr concentration is
substantially independent of the Pb ratio. In general, the
resulting PZT films become rougher beyond the self-correcting
regime where excess Pb is incorporated into the PZT film. It has
been observed that within the self-correcting lead composition
regime precursor vapor Pb/(Zr+Ti) (gas) ratios between 1.00 and
1.07 produce PZT films with optimal electrical properties.
Accordingly, during deposition, the precursor solution (liquid)
Pb/(Zr+Ti) ratio preferably is between 0.3 and 3.0 and, more
preferably, is between 0.8 and 1.3 and, still more preferably, is
between 1.00 and 1.07. Under these conditions, the precursor
solution (liquid) Zr/(Zr+Ti) ratio preferably is between 0.05 and
0.70 and, more preferably, between 0.30 and 0.40. In one
embodiment, the precursor solution (liquid) Zr/(Zr+Ti) ratio is
0.40, which translates to a Zr/(Zr+Ti) ratio of about 0.25 to 0.27
in the resulting PZT film. In the embodiments described above in
connection with Table 1, the precursor mixtures in source reagent
ampoules 28, 30 have the same Zr/(Zr+Ti) ratio, but have different
Pb/(Ti+Zr) ratios, with one source reagent ampoule having a
relatively high Pb/(Ti+Zr) ratio and the other source reagent
ampoule having a relatively low Pb/(Ti+Zr) ratio.
[0042] The baseline process yields 70 nm films with an RMS
roughness of 8 nm and a z-range of 58 nm, as measured by atomic
force microscopy (AFM). The dependence of RMS roughness on PZT film
thickness is illustrated in FIG. 6.
2TABLE 2 Heater Temperature 640.degree. C. Wafer Temperature
.about.600.degree. C. Pre-Deposition Time on Chuck 30 sec
on-pins/60 sec on-heater He Carrier Flow Through Vaporizer 250 sccm
Oxygen Flow 1000 sccm Ar Purge Flow 250 sccm Ar Push Gas Pressure
on Precursor 60 psi Ampoules Vaporizer Temperature 190.degree. C.
Jackets/Lid/Feedthrough Temperatures 190.degree. C. Showerhead to
Heater Spacing 350 mils Chamber Pressure 4 Torr Low Pb Precursor
Flow 65 to 82 mg/min High Pb Precursor Flow 118 to 135 mg/min Total
Precursor Flow 200 mg/min Pb/(Zr + Ti) (in liquid) 1.00 to 1.14
Zr/(Zr + Ti) (in liquid) 0.40 Deposition Rate .about.160 .ANG./min
Substrate Ir (100 nm)/Si.sub.3N.sub.4/SiO.sub.2/Si and IrO.sub.x
(50 nm)/Ir (50 nm)/Si.sub.3N.sub.4/ SiO.sub.2/Si
[0043] In addition to proper selection of the precursor solution
Pb/(Zr+Ti) ratios, other process parameters have been found to
improve the characteristics of the self-correcting Pb composition
regime, even at low processing temperatures. For example, it has
been discovered that the process pressure and the composition of
the oxidizing co-reactant gas have a substantial impact on the
range of the self-correcting Pb composition regime and,
consequently, on the degree to which the process temperature may be
reduced. In particular, a process pressure of 0.5-4 torr and, more
preferably 2 torr, and the addition to the oxidizing co-reactant
gas flow of N.sub.2O in a concentration of 5-100% and, more
preferably 50-75%--with the remaining portion of the co-reactant
gas being O.sub.2, in this embodiment--provides a PZT film
deposition process with a relatively large self-correcting Pb
composition regime, even at a wafer temperature of approximately
575.degree. C.
[0044] FIGS. 7, 8, 9 and 10 graphically illustrate the effects of
process pressure and N.sub.2O on the relative size of the
self-correcting Pb composition regime. As shown in FIG. 7, for a
constant wafer heater temperature of 630.degree. C., the
self-correcting Pb composition regime extends to lower Pb/(Ti+Zr)
(gas) values (and the range of the self-correcting regime
increases) as the pressure decreases. As shown in FIG. 8, the
self-correcting Pb composition regime extends to higher Pb/(Ti+Zr)
(gas) values (and the range of the self-correcting regime
increases) when N.sub.2O is added to the oxidizing co-reactant gas
flow. As shown in FIGS. 9 and 10, by combining a low process
pressure with an oxidizing co-reactant gas comprising N.sub.2O, the
range of the self-correcting Pb composition regime may be increased
significantly, improving the robustness of the deposition process.
Two exemplary baseline chemistry and process condition combinations
that incorporate these features are summarized below in Tables 3
and 4.
3TABLE 3 Heater Temperature 630.degree. C. Wafer Temperature
.about.586.degree. C. Pre-Deposition Time on Chuck 30 sec
on-pins/60 sec on-heater He Carrier Flow Through Vaporizer 250 sccm
Oxygen Flow 500 sccm N.sub.2O Flow 500 sccm Ar Purge Flow 250 sccm
Ar Push Gas Pressure on Precursor 60 psi Ampoules Vaporizer
Temperature 190.degree. C. Jackets/Lid/Feedthrough Temperatures
190.degree. C. Showerhead to Heater Spacing 350 mils Chamber
Pressure 2 Torr Low Pb Precursor Flow 82 to 115 mg/min High Pb
Precursor Flow 85 to 118 mg/min Total Precursor Flow 200 mg/min
Pb/(Zr + Ti) (in liquid) 0.79 to 1.00 Zr/(Zr + Ti) (in liquid) 0.40
Deposition Rate .about.107 .ANG./min Substrate Ir (100
nm)/Si.sub.3N.sub.4/SiO.sub.2/Si and IrO.sub.x (50 nm)/Ir (50
nm)/Si.sub.3N.sub.4/ SiO.sub.2/Si
[0045]
4TABLE 4 Heater Temperature 620.degree. C. Wafer Temperature
.about.575.degree. C. Pre-Deposition Time on Chuck 30 sec
on-pins/60 sec on-heater He Carrier Flow Through Vaporizer 250 sccm
Oxygen Flow 250 sccm N.sub.2O Flow 750 sccm Ar Purge Flow 250 sccm
Ar Push Gas Pressure on Precursor 60 psi Ampoules Vaporizer
Temperature 190.degree. C. Jackets/Lid/Feedthrough Temperatures
190.degree. C. Showerhead to Heater Spacing 350 mils Chamber
Pressure 2 Torr Low Pb Precursor Flow 82 to 115 mg/min High Pb
Precursor Flow 85 to 118 mg/min Total Precursor Flow 200 mg/min
Pb/(Zr + Ti) (in liquid) 0.79 to 1.00 Zr/(Zr + Ti) (in liquid) 0.40
Deposition Rate .about.96 .ANG./min Substrate Ir (100
nm)/Si.sub.3N.sub.4/SiO.sub.2/Si and IrO.sub.x (50 nm)/Ir (50
nm)/Si.sub.3N.sub.4/ SiO.sub.2/Si
[0046] After a PZT film having a desired thickness is deposited on
substrate 22 (step 81), the PZT precursor vapor again is diverted
to evacuation system 52 (step 82). Following deposition, substrate
22 remains within CVD chamber 12 for a post-deposition waiting
period before it is transported from CVD chamber 12 into a buffer
chamber. In one embodiment, the post-deposition waiting period is
about 5 seconds, or longer.
[0047] Other embodiments are within the scope of the claims. For
example, in some embodiments, a single source reagent ampoule
containing a complete, premixed solution of lead, titanium and
zirconium precursors may be used. In other embodiments, three or
more complete, premixed source reagent solutions may be mixed
together to define the metalorganic composition space.
[0048] Still other embodiments are within the scope of the
claims.
* * * * *